There exist a number of different input and output devices suitable for use in a human machine interface (HMI). A popular output device is the active matrix flat panel display.
FIG. 1 illustrates a flat-panel display device having a display matrix 2 and control circuitry 4 for controlling the display matrix. The display matrix 2 in this example is monochrome and comprises an N row by M column array of picture element (pixel) circuits 15nm, each comprising a pixel. A colour display is accomplished by dividing each pixel into sub pixels the number of which is the same as they number of primary colours (usually three for red, green, blue, RGB). The portion of the display matrix 2 corresponding to n=1, 2 and 3 and m=1, 2 and 3 is illustrated. Each of the N rows of pixel circuits 151m, 152m, 153m . . . 15Nm, where m=1, 2, 3 . . . M, has an associated row select line 21n. The row select line 21n is connected to each of the pixel circuits 15n1, 15n2, 15n3 . . . 15nM in its associated row. If the row select line is asserted the pixel circuits in the associated row are enabled. If the row select line is not asserted, the pixel circuits in the associated row are not enabled. Each of the M columns of pixel circuits 15n1, 15n2, 15n3 . . . 15nM, where n=1, 2, 3 . . . M, has an associated data line 20m. The data line 20m is connected to each of the pixel circuits 151m, 152m, 153m . . . 15Nm in its associated column. The pixel circuit 15nm is enabled by asserting the row select line 21n and the greyscale of a pixel (n,m) of an enabled pixel circuit 15nm is determined by either the voltage, current, or electrical charge provided via the data line 20m.
The control circuitry 4 comprises timing control circuitry 6, column driver circuitry 8 and row selection circuitry 10. The timing control circuitry 6 receives an input from a computer (not shown) which indicates the greyscale value of each pixel of the display matrix 2 for one display frame and provides an output to the column driver circuitry 8 and to the row selection circuitry 10.
To paint an image on the display matrix 2, the row select lines and data lines are successively scanned. The row selection circuitry 10 asserts the select line 211 and does not assert any other of the row select lines. The M pixel circuits 151m, where m=1, 2, 3 . . . , in first row of the display matrix 2 are thereby enabled. The column driver circuitry converts each of the greyscale values for the M pixels in row n provided from the computer to voltage values and applies the voltage to each of the M data lines 20m, where m=1, 2, 3 . . . . The voltage on a data line determines the greyscale of the enabled pixel associated with it. The selection circuitry asserts the select line 212 for the next row and the process is repeated. Thus one row of pixels is painted at a time and each row is painted in order until the frame is complete. The computer then provides the greyscale value of each pixels of the display matrix 2 for the next frame and it is painted one row at a time.
The display may be an active matrix (AM) or a passive matrix (PM) display. In the PM mode, the pixel greyscale is only maintained while its associated row select line is asserted. For example, if a PM has 240 rows, each row is only switched on during 1/240 of the frame period. For displays with high pixel count and therefore a large number of rows, the pixel switch-on time becomes shorter and the contrast and brightness is therefore reduced. To solve this problem AM was introduced. Each pixel now has a means for maintaining its greyscale after its scan i.e. when its associated row select line is de-asserted.
Reflective displays modulate the light incident on the display and transmissive displays modulate light passing through the display from a backlight. Transflective displays are a combination of reflective and transmissive displays and allow viewing in the dark as well as in bright sunlight. Liquid crystal displays (LCDs) are commonly used in these types of displays. LCDs form an image by reorienting liquid crystal (LC) molecules using an electric field. The reorientation causes the polarisation-rotating properties to change and combining this with polarisers can be used to switch pixels on and off. A matrix of LCD pixels is controlled by applying a voltage to a selected combination of a row and a column via the data lines 20.
FIG. 2 illustrates a portion of an active matrix LCD (AMLCD). The pixel circuits 15nm described in relation to FIG. 1 have been designated by the reference numerals 25nm in FIG. 2 to indicate that they are AMLCD pixel circuits. The figure illustrates a first pixel circuit 2511 connected to the first data line 201 and the first row scan line 211 and a second pixel circuit 2521 connected to the data line 201 and the second row scan line 212. The first and second pixel circuits are identical. The first pixel circuit 2511 comprises a first switching field effect transistor 221, a first liquid crystal picture element 231 having an inherent capacitance and a first storage capacitor 241. The gate of the first switching transistor 221 is connected to the first row scan line 211, its sources is connected to the first data line 201 and its drain is connected to a terminal of the first liquid crystal picture element 231 and to a plate of the first storage capacitor 241. The other plate of the first storage capacitor 241 is connected to the second row scan line 212. The first switching transistor 221 operates as a switch. When the first row scan line 211 is asserted the transistor conducts and when it is not asserted it does not conduct. Thus when the first row scan line 211 is asserted, the first storage capacitor 241 is charged by the voltage applied via the first data line 201 to set the greyscale of the first liquid crystal picture element 231. When the first row scan line 211 is no longer asserted the charged first storage capacitor 241 maintains the correct voltage across the first liquid crystal picture element 231 and maintains the correct greyscale. In this way, there is no reduction in contrast or brightness even for high-resolution displays.
The field effect switching transistors are normally thin film transistors (TFT) formed from semiconductors, in most cases hydrogenated amorphous silicon (a-Si:H) or low temperature polycrystalline silicon (p-Si). The data lines, scan lines, switching transistors and storage capacitors forming the display matrix can be integrated on a single substrate as an integrated circuit. The substrate is usually made from glass but increasingly also from plastics.
Emissive displays produce their own light. These types of displays include: field emission displays (FED); organic light-emitting diode (OLED) and thin-film electroluminescence displays (TFEL). While FEDs, OLEDs, and TFELs all can be passively driven, AM driving is preferred for the same reason as LCDs. The difference is that they are driven at constant current whereas LCDs rely on constant voltage. The intensity of the emitted light is controlled by current which, via the AM driving, is kept constant during one frame. It can also be controlled by the amount of charge via pulse-width modulation and constant current.
FIG. 3 illustrates a portion of a OLED active matrix display. The pixel circuits 15nm described in relation to FIG. 1 are designated by the reference numerals 35nm in FIG. 3 to indicate that they are OLED pixel circuits. The figure illustrates an exemplary emissive pixel circuit 3511 connected to the data line 201, the row scan line 211, a common anode 36 and a common cathode 37. The emissive pixel circuit 3511 comprises a switching field effect transistor 32, a light emitting diode 33, a storage capacitor 34 and a drive transistor 36. The gate of the switching transistor 32 is connected to the row scan line 211, its source is connected to the data line 201 and its drain is connected to a plate of the storage capacitor 34 and the gate of the drive transistor 36. The other plate of the storage capacitor 34 is connected to the common anode 36. The drain of the drive transistor is connected to the common anode 36 and the light emitting diode 33 is connected between the source of the drive transistor 36 and the common cathode 37.
The switching transistor 32 operates as a switch. When the first row scan line 211 is asserted the switching transistor 32 conducts and when it is not asserted it does not conduct. Thus when the first row scan line 211 is asserted, the voltage applied via the first data line 201 controls the current flowing through the drive transistor 36 (and hence the intensity of the LED 33) and charges the storage capacitor 34. When the first row scan line 211 is no longer asserted, the charged storage capacitor 34 maintains the correct voltage at the gate of the drive transistor 36 and thereby maintains the correct current through the LED 33 and thus the correct greyscale.
The field effect switching transistor and the first drive transistor 36 are normally thin film transistors (TFT) formed from semiconductors such as hydrogenated amorphous silicon (a-Si:H) or low temperature polysilicon (p-Si). The data lines, scan lines, switching transistors and storage capacitors forming the display matrix can be integrated on a single substrate as an integrated circuit.
It is desirable to use the display area provided by the flat panel display for optical input while it is being used for output. Thus far this has usually been achieved by using physically distinct touchscreen devices in combination with the flat panel display device. Resistive touchscreens are the most common touchscreens and comprise a glass or plastic substrate, an air gap with spacers and a flexible film. The opposing faces of the substrate and film are coated with a transparent electrode usually ITO. When touched the upper and lower surfaces are brought into contact and the resistances in the x and y direction are measured. These types of touch screens reduce the optical transmission from the underlying screen, introduce colour shift into a displayed image and may only have relatively small dimensions. Optical scattering against the spacer particles and the glass surface further reduces the image quality of the underlying display. Some of these disadvantages may be addresses by using more sophisticated, complex and costly touch screen technology. For example an optical touch screen may be used in which light is generated parallel to the display surface and a special pointing object touched on the display surface creates a shadow which is detected. However, this techniques requires expensive optical components such as lenses, mirrors and transmitters and has a limited resolution. Another technique detects surface acoustic waves travelling on a thick front glass, but this has limited resolution.
There therefore does not exist any satisfactory circuit which combines optical input with display output. The existing solutions may require extra components which add size, weight and expense. The existing solutions also suffer from insufficient resolution and if a touch screen is placed in front of the display it introduces parallax because the input and output planes are not co-planar and it reduces the image quality.